FORCE

When a hypothesis of a conditional rule has the form (force hyp) it
is logically equivalent to hyp but has a pragmatic effect. In
particular, when the rule is considered, the needed instance of the
hypothesis, hyp', is assumed and a special case is generated,
requiring the system to prove that hyp' is true in the current
context. The proofs of all such ``forced assumptions'' are delayed
until the successful completion of the main goal.
See forcing-round.

It sometimes happens that a conditional rule is not applied because
some hypothesis, hyp, could not be relieved, even though the
required instance of hyp, hyp', can be shown true in the context.
This happens when insufficient resources are brought to bear on hyp'
at the time we try to relieve it. A sometimes desirable alternative
behavior is for the system to assume hyp', apply the rule, and to
generate explicitly a special case to show that hyp' is true in the
context. This is called ``forcing'' hyp. It can be arranged by
restating the rule so that the offending hypothesis, hyp, is
embedded in a call of force, as in (force hyp). By using the
:corollary field of the rule-classes entry, a hypothesis
can be forced without changing the statement of the theorem from
which the rule is derived.

Technically, force is just a function of one argument that returns
that argument. It is generally enabled and hence evaporates during
simplification. But its presence among the hypotheses of a
conditional rule causes case splitting to occur if the hypothesis
cannot be conventionally relieved.

Since a forced hypothesis must be provable whenever the rule is
otherwise applicable, forcing should be used only on hypotheses that
are expected always to be true.

A particularly common situation in which some hypotheses should be
forced is in ``most general'' type-prescription lemmas. If a single
lemma describes the ``expected'' type of a function, for all
``expected'' arguments, then it is probably a good idea to force the
hypotheses of the lemma. Thus, every time a term involving the
function arises, the term will be given the expected type and its
arguments will be required to be of the expected type. In applying
this advice it might be wise to avoid forcing those hypotheses that
are in fact just type predicates on the arguments, since the routine
that applies type-prescription lemmas has fairly thorough knowledge
of the types of all terms.

Force can have the additional benefit of causing the ACL2 typing
mechanism to interact with the ACL2 rewriter to establish the
hypotheses of type-prescription rules. To understand this remark,
think of the ACL2 type reasoning system as a rather primitive
rule-based theorem prover for questions about Common Lisp types,
e.g., ``does this expression produce a consp?'' ``does this
expression produce some kind of ACL2 number, e.g., an integerp, a
rationalp, or a complex-rationalp?'' etc. It is driven by
type-prescription rules. To relieve the hypotheses of such rules,
the type system recursively invokes itself. This can be done for
any hypothesis, whether it is ``type-like'' or not, since any
proposition, p, can be phrased as the type-like question ``does p
produce an object of type nil?'' However, as you might expect, the
type system is not very good at establishing hypotheses that are not
type-like, unless they happen to be assumed explicitly in the
context in which the question is posed, e.g., ``If p produces a
consp then does p produce nil?'' If type reasoning alone is
insufficient to prove some instance of a hypothesis, then the
instance will not be proved by the type system and a
type-prescription rule with that hypothesis will be inapplicable in
that case. But by embedding such hypotheses in force expressions
you can effectively cause the type system to ``punt'' them to the
rest of the theorem prover. Of course, as already noted, this
should only be done on hypotheses that are ``always true.'' In
particular, if rewriting is required to establish some hypothesis of
a type-prescription rule, then the rule will be found inapplicable
because the hypothesis will not be established by type reasoning
alone.

The ACL2 rewriter uses the type reasoning system as a subsystem. It
is therefore possible that the type system will force a hypothesis
that the rewriter could establish. Before a forced hypothesis is
reported out of the rewriter, we try to establish it by rewriting.

This makes the following surprising behavior possible: A
type-prescription rule fails to apply because some true hypothesis
is not being relieved. The user changes the rule so as to force the
hypothesis. The system then applies the rule but reports no
forcing. How can this happen? The type system ``punted'' the
forced hypothesis to the rewriter, which established it.

Finally, we should mention that the rewriter is never willing to force when
there is an if term present in the goal being simplified. Since
and terms and or terms are merely abbreviations for if
terms, they also prevent forcing. Note that if terms are ultimately
eliminated using the ordinary flow of the proof (but
see set-case-split-limitations), allowing force ultimately to function
as intended. Moreover, forcing can be disabled, as described above; also
see disable-forcing.